The present invention generally relates to semiconductor processing and, more particularly, to a system and method to monitor reticle heating temperature during fabrication.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller features sizes are required. This may include the width and spacing of interconnecting lines.
The requirement of small features with close spacing between adjacent features requires high resolution lithographic processes. In general, projection lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as light, x-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the mask or reticle, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through a transparency causes the image area to become selectively cross-linked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (e.g., uncross-linked) or deprotected areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. A recognized way of reducing the feature size of circuit elements is to lithographically image them with radiation of a shorter wavelength. xe2x80x9cLongxe2x80x9d or xe2x80x9csoftxe2x80x9d x-rays (e.g., extreme ultraviolet (EUV)) are now at the forefront of research in an effort to achieve the smaller desired feature sizes.
EUV lithography may be carried out as follows, EUV radiation is projected onto a resonant-reflective reticle. The resonant-reflective reticle reflects a substantial portion of the EUV radiation which carries an IC pattern formed on the reticle to an all resonant-reflective imaging system (e.g., series of high precision mirrors). A demagnified image of the reticle pattern is projected onto a resist coated wafer. The entire reticle pattern is exposed onto the wafer by synchronously scanning the mask and the wafer, such as during step-and-scan exposure.
Although EUV and other short wavelength lithography provides substantial advantages with respect to achieving high resolution patterning, errors may still result from the lithography process. For instance, the reflective reticle employed in the lithographic process is not completely reflective and consequently will absorb some of the radiation. The absorbed radiation results in heating of the reticle. As the reticle increases in temperature, mechanical distortion of the reticle may result due to thermal expansion of the reticle. Such mechanical distortion of the reticle can manifest in overlay errors. In photolithography, overlay is defied as layer-to-layer registration performance. For example, silicon is a material which may be used as a substrate for a reticle, and silicon has a coefficient of thermal expansion of approximately 2 ppm/xc2x0 C. Across a typical 100 mm image field, a 0.5xc2x0 C. deviation in temperature can result in a registration error of 100 nm, in circumstances where  less than 10 nm is desired.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the present invention provides a system to monitor temperature of a substrate, such as a mask, reticle or wafer. The system includes one or more temperature sensors operative to sense temperature of the substrate. A control system receives temperature information based on the sensed temperature and provides a control signal based on the temperature information. In a particular aspect, an exposing source exposes the substrate based on the control signal. By way of example, the temperature information can be collected during one or more exposure cycles, such that the control system can control exposure time of the substrate for subsequent exposure cycles based on the temperature information previously collected for that substrate.
Another aspect of the present invention provides a method for monitoring and/or regulating substrate temperature. The substrate is exposed with radiation, such as light of a desired wavelength. The temperature of the substrate is sensed. One or more parameters associated with the exposure of the substrate can then be controlled based on the sensed temperature, which, for example, can include the sensed temperature of the substrate associated with the current and/or previous exposure cycles.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.